The heat-responsive erasable toner containing a color-forming compound and a color-developing agent disclosed in U.S. Pat. No. 7,354,885 is a known example of erasable toner. In this technique, the color-forming compound and the color-developing agent are incorporated into toner by being melted and kneaded with a binder resin using a kneading and pulverization method to provide an erasable toner. The printed paper formed with the erasable toner is heated at 100 to 200° C. for about 1 to 3 hours to decolor the printed portion, and the decolored paper can be reused. By thus reducing paper consumption, the technique contributes to reducing the environmental load.
However, because the kneading and pulverization method involves high-shear kneading at high temperatures of about 100 to 200° C., the leuco dye and the color-developing agent are uniformly dispersed in the binder, resin to obstruct the reaction between the leuco dye (color-forming compound) and the color-developing agent, and lower the developed color density of the toner. Further, if the toner materials, such as a binder resin and a release agent, have a decoloring action, the color density of the toner is lowered during the kneading, thus requiring the use of toner materials having a weak decoloring action. The binder resin is particularly troublesome or problematic in this regard, because only a specific resin with no decoloring action such as styrene-butadiene resin can be used, and it is very difficult to use polyester resin or styrene acryl resin, which, despite superior fusibility, is likely to exert decoloring action.
Instead of the kneading and pulverization method, the present inventors have proposed a “wet” method, in which erasable colored fine particles, and binder resin or other fine particles are aggregated and fused in an aqueous medium to produce a toner (US2010/209839A1). Because this method aggregates the fine particles, a toner of a small particle diameter can be produced, and the shape of the particles can be varied from the potato shape to spherical by varying the conditions of the fusing heat treatment. Further, erasable fine particles can be produced as a mixture with the binder resin, without application of mechanical shear or high heat history according to melting and kneading. For example, a toner can be produced at relatively low temperatures of below 80° C. and above the Tg of the binder resin. The method is therefore effective for toners comprising encapsulated erasable colored fine particles, or for toners that are erased irreversibly by heat history.
However, it is very difficult to fully incorporate the encapsulated erasable fine particles into the toner, because such fine particles have a very high dispersion stability. As a result, fine particles of the toner are liable to be increased to deteriorate the image quality.